The invention relates to a method of implementing downlink frequency hopping to the frequency of a broadcast control channel in a radio system comprising a network part, a subscriber terminal and a radio connection as the transmission path between the network part and the subscriber terminal.
Frequency hopping was developed for military radio systems to complicate eavesdropping and to eliminate interference. In eliminating interference it should be noted that frequency diversity and interferer diversity are achieved by frequency hopping. Frequency hopping is implemented by a transmitter transmitting a given amount of radio information at a given frequency. The transmitter then continues to transmit radio information at another frequency. The frequency can be changed several times. Usually the frequency is changed as a known series, and this is called a frequency hopping sequence.
In radio systems, each base station has one carrier, called a broadcast channel. The term control channel may also be used. In fact, the term refers to several channels used by a subscriber terminal and the network part for controlling their operations. Downlink broadcast channels are called broadcast control channels. In addition to these, the same carrier is used to transmit the actual traffic channels, on which user payload, such as speech or data, are transferred after a connection has been established. When needed, signalling data may also be transported within traffic channels.
The term channel is used in two different meanings. In this representation, a channel refers to a logical channel containing data characteristic of it. The second meaning of a channel refers to a physical channel used for carrying the logical channel. In this representation, a physical channel is called a time slot.
In the GSM system, one physical channel is one time slot of a TDMA frame. Logical channels are common channels or dedicated channels. A TDMA frame comprises 8 time slots. A 26-multiframe of the length of 26 TDMA frames has been defined for dedicated channels. Similarly, a 51-multiframe of the length of 51 TDMA frames has been defined for common channels.
The common channels comprise BCH channels (Broadcast Channel) and CCCH channels (Common Control Channel). The BCH channels comprise a FCCH channel (Frequency Correction Channel), a SCH channel (Synchronization Channel) and BCCH channels (Broadcast Control Channel). The CCCH channels comprise a PCH channel (Paging Channel), an AGCH channel (Access Grant Channel) and RACH channels (Random Access Channel). The dedicated channels comprise traffic channels and DCH channels (Dedicated Control Channel). The DCH channels comprise an SDCCH channel (Stand-alone Dedicated Control Channel), an SACCH channel (Slow Associated Control Channel) and an FACCH channel (Fast Associated Control Channel).
As to the present invention, the essential channels are the broadcast control channels, including the FCCH channel, SCH channel, BCCH channels and CCCH channels (the PCH and AGCH channels). The FCCH channel comprises frequency correction information for a subscriber terminal. The SCH channel comprises frame synchronization information. The BCCH channel comprises general base station-dedicated information. Of the CCCH channels, the PCH channel comprises call information and the AGCH channel information about the allocation of a traffic channel to a subscriber terminal.
The broadcast control channel is transmitted at full transmitter power and at the same frequency, i.e. frequency hopping cannot be utilized. This causes problems to planning and using a radio system.
The major problem is double network planning. At first, the use of normal carriers is planned. It can be enhanced by frequency hopping, discontinuous transmission and power control. Secondly, the use of the carriers of the broadcast control channels is planned.
Another major problem is that the capacity of the system reduces as the traffic channels that are on the same carrier as the broadcast control channel cannot utilize frequency hopping, discontinuous transmission and power control.
It is the object of the present invention to implement frequency hopping to the frequency of the broadcast control channel, the method avoiding the presented problems.
This is achieved with the method described in the preamble, characterized in that the method comprises the following steps:
A) the network part transmits on the broadcast control channel a first signal group to the subscriber terminal at a given frequency, the first signal group stating at which second frequency the first signal group will be transmitted the next time,
B) the network part then transmits on the broadcast control channel a known number of other signals to the subscriber terminal using frequency hopping, and the offset of the first frequency hop has been agreed in advance, and the offsets of the other frequency hops are agreed in advance,
C) step A is entered, the network part transmitting the following first signal group to the subscriber terminal at a second frequency,
D) steps A to C are repeated, the frequency hopping sequence being formed of a given number of steps A to C.
The invention also relates to a system for implementing downlink frequency hopping to the frequency of a broadcast control channel in a radio system comprising a network part, a subscriber terminal and a radio connection as the transmission path between the network part and the subscriber terminal.
The system is characterized in that the network part is adapted to transmit on the broadcast control channel a first signal group to the subscriber terminal at a given frequency, the first signal group stating at which second frequency the first signal group will be transmitted the next time, and to transmit on the broadcast control channel other signals to the subscriber terminal using frequency hopping, the offset of the first frequency hop being agreed in advance, and the offsets of the other frequency hops being agreed in advance; the subscriber terminal is adapted to generate a frequency hopping sequence of the transmission frequencies of the first signal group, the offset of the first frequency hop and the offsets of the other frequency hops.
The method of the invention provides a plurality of advantages. The performance of all channels on the carrier of the broadcast control channel improves significantly as frequency hopping can be used.
A significant advantage is gained by network planning becoming much easier. Double network planning is no longer needed, as all the channels transmitted by the network part are able to utilize frequency hopping. It is even possible that cell frequency planning does not have to be carried out at all: the base station randomly selecting a frequency hopping sequence. Should some neighbouring base stations use the same frequencies, frequency hopping ensures that the performance remains sufficiently good.
The system of the invention has the same advantages as described above in connection with the method. The preferred embodiments of the invention and the other more detailed embodiments emphasize the advantages of the invention. It is obvious that the preferred embodiments and the more detailed embodiments can be combined to different combinations to achieve the desired technical power.